1. Field of the Invention
The present invention relates generally to a method and device for detection of direct sequence spread spectrum (DSSS) signals using pseudo-random noise (PN) modulation when offsets are present in the carrier frequency.
2. Description of the Related Art
Wireless communications systems based upon direct sequence spread spectrum (DSSS) signals, including code division multiple access (CDMA), use a common carrier frequency band for communication with all base stations within a system. The carrier signal is modulated by a signal generated by a pseudo-random noise (PN) generator, which provides means for discrimination from the common frequency band. The PN spreading code contained in this second signal consists of a sequence of binary chips, each having a chip period. The combined carrier and PN signals are modulated by a third signal comprising a digitized voice or data signal.
In order to convert the third signal for use at the receiver, after carrier frequency demodulation, a local PN generator in the receiver must be synchronized to the incoming PN sequence. The PN codes are used for both initial acquisition of the signal and for data transmission. By removing the PN sequence from the received signal and integrating it over a symbol period, a despread signal can be obtained. The despreading of the received signal is accomplished by generating a local replica of the PN code in the receiver, then synchronizing the local PN code to the PN code which is superimposed on the received waveform contained in the received signal. Multiplication or remodulation of the incoming signal by the synchronized local PN code replica produces the desired despreading.
In the acquisition phase, the spreading signals are brought into alignment with each other. It is during this phase that the receiver must perform some of its most critical functions, which are to establish chip timing synchronization and correct for frequency offset. Once synchronization is attained, a closed loop tracking system within the receiver clock must be continuously adjusted in frequency and phase in order to optimize the sampling of the data signal extracted from each received signal.
The process of synchronizing the local and received PN signals is typically performed in two stages. Initially, a coarse alignment of the two PN signals is produced to within a small residual timing offset to achieve PN acquisition. Once acquired, the PN codes must be maintained in fine synchronization in a process known as PN tracking. In direct sequence CDMA systems, the PN codes are often very long. To minimize the receiver hardware complexity that would be required for correlation of the full PN code, correlation over a partial period of the PN code, i.e., the "dwell time", is used.
Detection of DSSS signals for acquisition is typically achieved using a receiver similar to the configuration shown in FIG. 1, which consists of a correlator 102 for comparing the received signal plus noise with a local replica of the PN code from the local PN generator 104, an integrator 106 for integrating the detector output for a fixed dwell time to find the total integration power, a square law envelope detector 108, and a comparator 110 for comparing the power to a pre-set threshold. When the power level exceeds the threshold, the coarse alignment of the PN signal has been achieved. The optimal threshold level which is used to determine whether acquisition has occurred is not related to a fixed value, but is instead a function of the signal-to-noise ratio (SNR). As is known, the SNR of a communication channel will vary as a function of time as well as the velocity and location of the receiver.
A simple exemplary acquisition technique uses a maximum likelihood approach with single dwell time. This technique requires the received PN code signal to be correlated with all possible phase positions of the local PN code. The correlations are performed in parallel and the corresponding detector outputs all pertain to the identical observation (dwell) of the received signal (plus noise). The correct PN alignment is determined by a comparator as being the local PN code phase position which produces the maximum output from the detector. The acquisition can be accomplished rapidly due to the parallel operation. However, for the lengthy codes utilized in CDMA signals and the large processing gains required, the complexity of the parallel computation is prohibitive. Other acquisition techniques are known in the art. A brief description of some of these techniques is provided in U.S. Pat. No. 5,440,597 of Chung, et al, the subject matter of which one of the present inventors is also a co-inventor. The disclosure of the '597 patent is incorporated herein by reference.
When frequency error exists between the transmitter and receiver, due either to multi-path transmission (Rayleigh fading), temperature-induced oscillator drift, Doppler effect, or other distortion-producing phenomenon, or a combination thereof, the receiver/demodulator must first determine the frequency offset before acquisition can occur. The presence of frequency offsets cause the relative code phase between the received and locally generated PN codes to be time varying. More significantly, the offsets effect the average search rate.
When detecting a signal that has passed through an Additive White Gaussian Noise (AWGN) channel, the detection time is a function of the noise variance only. The greater the noise variance, the longer the time required for correlation, leading to longer detection times. Since frequency offsets also increase the detection time, the presence of a frequency offset in an AWGN channel can cause what might otherwise be acceptable detector performance to experience significant degradation. The ability to coherently detect the signal from an AWGN channel may be lost due to the presence of frequency offset.
A common approach to alleviate the frequency offset problem, and deal with the lengthy PN codes, is to segment the total integration time into a number of smaller partial correlations, or sub-dwells (observation periods of the received signal), to reduce the loss resulting from the frequency term. The magnitude is computed for each of the sub-dwells, and the magnitudes for all of the sub-dwells are summed to obtain a total integration magnitude. This type of acquisition system is known as the "non-coherent addition method", which is a type of multiple dwell serial PN acquisition system. (See, e.g., M. K. Simon, et al., Spread Spectrum Communications Handbook, Revised Edition, 1994, McGraw-Hill, Inc., Part 4, Ch. 1, "PseudoNoise Code Acquisition in Direct-Sequence Receivers", incorporated herein by reference.) The resulting total magnitude in this method is significantly reduced, since the sub-dwells represent only a fraction of the dwell time, making this approach feasible only when small frequency offsets are present, or short integration times are possible. Another disadvantage of this method is that coherent detection cannot be attained.
One proposed system for determining frequency offset is disclosed in U.S. Pat. 5,556,202 of Lang, which is incorporated herein by reference. In this system, a split correlator channel includes a pair of shift register strings with the corresponding stages of each string being of equal lengths. The received signal is partitioned between the stages. A phase rotator is provided between consecutive stages of the shift register strings, distributing the in-phase (I) and quadrature (Q) components of the received signal to the appropriate shift register string. Each shift register string stage is correlated with the selected PN sequence, and the results are summed and compared to the pre-set threshold for correlation. Assumptions are made with regard to phase rotation which can introduce errors. Further, this system still uses the non-coherent addition method, retaining certain disadvantages of such systems.
The needs remains for a system for rapidly determining frequency offsets for PN signal acquisition without relying on non-coherent methods. The system of the present invention addresses this need.